In general, a color filter is used for a liquid crystal display (LCD), an optical filter for a camera, and the like. A color filter can be fabricated by coating a fine region colored with more than three colors on a charge coupled device or a transparent substrate. Colored thin films can be commonly fabricated using dyeing, printing, electrophoretic deposition (EPD), pigment dispersion, and inkjet methods.
Dyeing methods form a colored film by forming an image with a dyeing agent such as a natural photosensitive resin such as gelatin and the like, an amine-modified polyvinyl alcohol, an amine-modified acrylic-based resin, and the like on a substrate, and then dyeing the image with direct dyes. However, the dyeing process may become more complex and lengthy, since it should include resist printing whenever a color needs to be changed to form a multicolored thin film on the same substrate. In addition, commonly used dyes and resins themselves generally can have good color vividness and dispersion but poor light fastness, water resistance, and heat resistance, which are very important characteristics.
Printing methods form a colored thin film by printing an ink prepared by dispersing a pigment into a thermally curable or photocurable resin and curing it with heat or light. This method may decrease material costs compared with other methods, but it can be difficult to form a fine and exact image, and also acquire a uniform thin film layer. Korean laid-open patent Nos. 95-703746 and 96-11513 disclose a method of making a color filter in an inkjet method. However, these methods also have the same problems of deteriorated durability and heat resistance as dyeing methods, since these methods also use a dye-type color resist composition dispersed from a nozzle to accomplish fine and precise color printing.
Korean laid-open patent Nos. 93-7000858 and 96-29904 disclose electrophoretic deposition (EPD) using an electric precipitation method. The electrophoretic deposition (EPD) can form a precise colored film that has excellent heat resistance and light fastness of the colored film, since it includes a pigment. However, when a finer electrode pattern is needed for a more sophisticated pixel in the future, it may be difficult to use electrophoretic deposition to make the same since a colored film may be stained or thicker at both ends due to electrical resistance.
Pigment dispersion methods form a colored film by repeating a series of processes such as coating, exposing to light, developing, and curing a photopolymer composition including a coloring agent on a transparent substrate with a black matrix. Pigment dispersion methods can improve heat resistance and durability, which are very important characteristics of a color filter, and uniformly maintain the thickness of the film. In addition, pigment dispersion methods are generally used since it is not only easy to apply but can also accomplish a fine pattern. As examples, Korean laid-open patent Nos. 92-7002502 and 95-7000359 and Korean patent publication Nos. 94-5617 and 95-11163 disclose a method of making a color resist using a pigment dispersion method.
However, it can be difficult to manage yield rate using pigment dispersion methods, since each color of red (R), green (G), and blue (B) respectively requires coating, exposure, development, and curing to form a pixel, which makes the whole process line longer and increases control factors among the processes. In addition, as electronic products, such as monitors, TVs, and the like, require a high color resolution rate and a high contrast ratio, pigment dispersion methods may exhibit several problems such as increasing the thickness of a coating layer and the like.
Accordingly, conventional pigment dispersion methods have recently been replaced with several new methods, such as inkjet printing methods. Inkjet printing methods form a light proof layer such as a black matrix and the like on a glass substrate and implants ink in the pixel space. This method does not require processes such as coating, exposure, development, and the like, and can thereby decrease necessary materials required for the processes and simplify the whole process line.
Inkjet methods should form a color layer among pixels to have uniform and excellent color characteristics. Accordingly, a nozzle should not be clogged when ink is dispersed from its head. Further, the ink should be dispersed with the same amount and the same number of drops inside a pixel. The dispersion performance critically depends on interaction between the surface of the head nozzle and the color ink. In addition, when the ink is ejected, it should be prevented from overflowing into the opening of neighboring pixels and contaminating them.
Furthermore, when a pattern is formed in the inkjet method, it should have the same reliability as in a pigment dispersion method. It should also have heat resistance, chemical resistance, film strength, storage stability, and the like. When the pattern does not satisfy these characteristics, it may cause a problem of color change, damage, and the like during post-processing of a color filter.